In recent years, mobile internet protocol (IP) in mobile communication has been widely examined (refer to Patent Document 1, below). Many of today's mobile devices communicate with one another using IP. To provide mobility support for mobile devices, “Mobility Support in IPv6” (refer to Non-patent Document 1, below) is being discussed in the Internet Engineering Task Force [IETF]. Here, movement of a mobile node (MN) between networks will be described using a portion of a conventional communication network configuration shown in FIG. 21. At the moment, the MN is connected to an access point (AP) 1 of an access router (AR) 1 and is moving towards an AP 2 within the AR 1. The AR 1 has a Prefix 1. An AR 2, described hereafter, has a Prefix 2.
As shown in FIG. 21, when the MN performs a handover (HO) from the AP 1 to the AP 2 within the AR 1, the MN performs a HO in a layer 2 (L2). On the other hand, when the MN performs a HO from the AP 2 within the AR 1 to an AP 3 within the AR 2, HO processing in the L2 and a layer 3 (L3) occurs. In other words, when the MN moves between networks having different prefixes, a HO processing in the L2 and a HO processing in the L3 occur. A L1 is not stable during a HO. Because the HO processing is performed in the L2 and in the L3, a long waiting time is required.
There is a well-known resolution method for preventing the HO processing in the L2 and in the L3 from occurring simultaneously. A portion of a network configuration in this method is shown in FIG. 22. As shown in FIG. 22, an area of the AR 1 and an area of the AR 2 overlap such that the HO processing in the L2 and in the L3 does not occur simultaneously. As a result of a configuration such as this, the HO processing in the L2 and the HO processing in the L3 occur separately and independently. The waiting time for the HO is shortened. The HO processing in the L1 does not occur in the overlapping area. Therefore, instantaneous interruption in the L1 stops.
However, in this case, a following problem occurs. When approaching the area configured by the AR2, the MN begins to see (confirm) the prefix 2 of the AR 2. With this serving as a trigger for the HO in the L3, the MN decides to perform the HO to the Prefix 2 at the AP 3. However, if the MN returns (makes a U-turn) to the AP 2 side immediately after making the decision, the HO processing in the L2 and the HO processing in the L3 occur simultaneously. As described above, this means that the waiting time for the HO increases. Therefore, an increase in the overlapping area between the areas respectively configured by the AR 1 and the AR 2 can be considered. As shown in FIG. 23, as a result of the overlapping area being increased, the HO processing in the L2 and the HO processing in the L3 can be further prevented from occurring simultaneously.    Patent Document 1: Japanese Patent Application Publication No. 2005-167388 (Paragraph 0016)    Non-patent Document 1: R. Hancock, et al., “Next Steps in Signaling: Framework”, RFC 4080, June 2005
However, even in this case, because the only trigger for the HO of the MN in the L3 is that the prefix is appeared (confirmed), the HO processing in the L3 may fortunately be completed in time before the HO in the L2 starts. However, the HO processing in the L3 may unfortunately not be completed in time before the HO in the L2 starts. The HO processing in the L2 and the HO processing in the L3 may occur simultaneously.